1. Field of the Invention
The present invention relates to a fixing device for fixing toner to a sheet and an image forming apparatus for forming and fixing a toner image on and to a sheet.
2. Description of the Related Art
An electrophotographic copier, printer, facsimile machine or a complex machine provided with functions of these may be illustrated as an image forming apparatus. Such an image forming apparatus is provided with a fixing device for fixing toner to a sheet. A certain type of the fixing device is provided with a fixing roller including a heater and a pressure roller to be pressed into contact with the fixing roller. A sheet having a toner image formed by an electrophotographic process transferred thereto is passed through a nip defined between the fixing roller and the pressure roller. In the nip, the toner on the sheet is melted to fix the toner image to the sheet. In order to control the temperature of the fixing roller, this type of the fixing device includes a temperature detector.
Japanese Unexamined Patent Publication No. H05-119664 discloses a fixing device suitably used to fix toner to a laminated sheet (e.g. envelope). In the fixing device disclosed in Japanese Unexamined Patent Publication No. H05-119664, a pressing force between a fixing roller and a pressure roller is variable and reduced while a laminated sheet passes. Meanwhile, this fixing device reduces a conveying speed of the laminated sheet and the temperature of the fixing roller. In this way, this fixing device creates an equivalent condition for fixing toner to the laminated sheet to that for fixing toner to one sheet. As a result, the fixing device disclosed in Japanese Unexamined Patent Publication No. H05-119664 may fix the toner to the laminated sheet without wrinkles or lateral displacement of the sheet due to a difference in circumferential speed (linear speed) between the fixing roller and the pressure roller while the laminated sheet passes between the fixing roller and the pressure roller. On the other hand, toner may be fixed to one sheet using a high pressing force, wherefore the warm-up time of the fixing roller can be shortened and the fixing device can be miniaturized.
A complicated mechanism for automatically switching a pressing force generally results in a high production cost and a high failure rate. A mechanism for manually switching a pressing force is inconvenient to a user. For example, if one user tries to print an ordinary copy sheet after another user printed some image on an envelope, he/she has to operate an image forming apparatus to switch the pressing force before he/she starts his/her printing.